Method for using electromagnetic grounded antennas as directional geophones

ABSTRACT

A method of seismic prospecting using electromagnetic grounded antennas to detect electromagnetic waves that are produced from acoustic waves in the earth&#39;s formation. Seismic waves reflected by a formation in the earth are converted into electromagnetic waves in the vicinity of the antenna according to the streaming potential theory. The antenna has two electrodes which detect the horizontal component of the electromagnetic waves, thus providing additional seismic information that is not readily available using standard geophones. Antennas are also not subject to coupling problems and thus provide more accurate information than traditional geophones. For example, using multicomponent detection, all three components of the seismic pressure gradient can be detected. In addition, using geophones and antennas directly provides a method of separating source and receiver-generated static corrections for more effective stacking of seismic data and allows computation of seismic signal velocity through the low-velocity layer.

FIELD OF THE INVENTION

The present invention relates to the acquisition of seismic data, andmore particularly to the use of electromagnetic grounded antennas asdirectional geophones for improved detection of seismic waves.

DESCRIPTION OF THE RELATED ART

Conventional seismic prospecting is usually accomplished by generatingacoustic waves from one or more seismic sources and then detectingreflections of those waves by interfaces in the earth formation beinganalyzed. Energy, usually in the form of an impulse from an explosion,is introduced into the ground at or near the surface. Spreading out fromthe source, the acoustic or seismic waves encounter discontinuities inthe physical properties of the rocks comprising the earth.Discontinuities of exploration interest are generally interfaces betweendifferent types of rock or formations. Upon encountering thesediscontinuities, referred to as reflectors, the seismic waves arepartially reflected back to the surface where they are detected andrecorded. The time required for the reflected energy to return indicatesthe depth of a reflector. Plotting this time for each detected signalwhile moving along the surface, one can assemble a picture of the rocklayers below from shallow depths to the deepest interface from whichreturning energy is measurable.

A common seismic source for reflection seismology is dynamite placed ator below the earth's surface. In many instances, the source is placed atthe bottom of a water-filled hole drilled through the weathered layer.The weathered layer, also referred to as the low-velocity layer (LVL),is defined as the soil and rock layer near the surface. The weatheredlayer is generally poorly consolidated and absorbs the energy of theseismic source and thus is avoided whenever possible. Accordingly, manytimes the seismic source is placed below the weathered layer to reduceattenuation of the generated seismic waves. The weathered layer alsoattenuates reflected seismic waves traveling back to the surface,removing many of the high frequency components of the reflected seismicwaves.

An explosion generates in solids both compressional waves in which themotion is substantially in the direction of travel and shear waves inwhich the motion is perpendicular to the compressional wave traveldirection. Compressional waves are reflected by interfaces separatingformations with different impedances, where a formation's impedance isthe product of the formation's density and the compressional wavevelocity. The amplitude of a reflected compressional wave isproportional to the difference of the impedances across an interface.Exploration seismology has traditionally analyzed reflectedcompressional waves, while the shear component of reflected seismicwaves has generally been used less frequently. Detectors used in landexploration, called geophones, measure the vertical and/or horizontalcomponent of the seismic wave, which is the time derivative of thedisplacement of the surface.

In summary, seismic energy reflected from a formation in the earthduring seismic prospecting has both vertical and horizontal componentscorresponding to compressional waves and shear waves, respectively, andeach of these components contain seismic information that can helpevaluate a formation. As is well known in the art, both thecompressional and shear components of any substance are necessary inorder to fully understand the elastic properties of the substance. Forexample, both compressional wave velocity and shear wave velocityprovide information on the elastic properties of a formation, which, forexample, may then be used to determine if gas exists in the pore spaceof the formation. However, conventional geophones that measure shearwaves are often poorly coupled to the surface of the earth, whichattenuates the high frequency shear energy. In other words, while ageophone is sensing a reflected seismic signal, vibrations from theseismic waves tend to affect the operation of the geophone. Also, thehigh frequency components of the seismic wave detected by a geophone aresubject to attenuation due to the weathered layer. Therefore, animproved method and apparatus is desired which detects the shear orhorizontal components of seismic energy received during seismicprospecting.

Background on the use of antennas in seismic prospecting is alsoappropriate. It is known in the art that antennas may provideinformation similar to that obtained by geophones in detecting thedirect arrival of seismic waves. The idea that antennas could be used asgeophones dates back to the 1930's. However, the use of antennas was notseriously considered and their use was dismissed because they wereconsidered to be too insensitive and highly variable frompoint-to-point. Consequently, antennas have not been used in practicalseismic applications. For example, in Field Experiments on theElectroseismic Effect, IEEE Transactions on Geoscience Electronics, Dec.1963, p. 23, the author concludes that the detection of seismic eventsthrough the conversion of seismic signals to electric signals was notsufficiently reliable and thus could not be recommended. U.S. Pat. Nos.2,054,067 and 2,156,259 each disclose the use of antennas as geophoneswhereby an array of antennas is used to detect seismically inducedresistance modulation of the subsurface. Each of these patents teachthat this arrangement preferentially detects vertically arriving P-waves(compressional waves) and discriminates against S-waves (shear waves).Thus, these patents teach that the shear wave component is not receivedby an antenna.

Background on electroseismic prospecting is also deemed appropriate.Various seismic techniques in addition to traditional acoustical wavedetection described above have been used in seismic exploration. Onetechnique that has been recently developed is referred to aselectroseismic prospecting or ESP. A method and apparatus for performingESP is described in U.S. Pat. No. 4,904,942. In ESP, seismic energyapplied to the earth by an explosion or seismic blast is converted intoelectromagnetic energy when mobile, conducting fluids are encountered ina formation. This EM energy is radiated back to the surface where it isdetected and analyzed. As described in U.S. Pat. No. 4,904,942, a theoryreferred to as the "streaming potential" theory explains the conversionof seismic energy into electromagnetic energy within an earth'sformation. This theory effectively analyzes what occurs when a seismicwave impacts a porous lithological formation causing fluid movement inthe formation and is most pronounced when at least two immisciblefluids, such as oil and water or gas and water, are present in theformation. The phenomenon also exists in the presence of a lithologicalstructure of high permeability where there is pore fluid in thestructure.

In accordance with this theory, a molecular bond attraction existsbetween the fluid and the porous surface of the solid formation. Thesebonds are distorted or broken by the rapid movement of the fluid uponcontact by an acoustical wavefront, thereby inducing in a dipole manneran electromagnetic response. This electromagnetic response produces EMwaves which radiate back to the earth's surface and which can bedetected and analyzed. The fluid movement accompanying a seismicpressure gradient is described by M. A. Biot in papers published by theJournal of the Acoustical Society of America in 1956 and 1962, at page168 of volume 28 and page 1254 of volume 34, respectively. Others, suchas J. O. Bockris and A. K. N. Reddy, have experimented with thestreaming potential and reported circa 1973 on their findings. Theapplication of the streaming potential theory in electroseismicprospecting is described in U.S. Pat. No. 4,904,942, previouslyreferenced.

SUMMARY OF THE INVENTION

The present invention comprises a method and apparatus for operating oneor more electromagnetic grounded antennas as directional geophones inseismic prospecting. Applicants have discovered that, since antennas arepreferentially selective in the direction in which they are aligned, EMgrounded antennas detect the shear component of the seismic wave andthus provide useful seismic polarization information. In addition,Applicants have discovered that antennas do not have the horizontalcoupling problems associated with standard geophones. Also, antennas areuseful in reducing static and noise effects in shear wave prospectingand allow direct measurements of seismic wave velocity and attenuationthrough the LVL.

An antenna comprising at least two electrodes is placed at or below theground and is used to measure the induced voltage resulting fromreflected seismic waves. Reflected acoustic or seismic energy in theneighborhood of the antenna near the subsurface is converted into anelectromagnetic field through the streaming potential effect. Theantenna is sensitive to the component of the seismic pressure gradientin the direction along the antennas' axis, i.e., the horizontalcomponent, and integrates this pressure gradient over the length of theantenna. This selectivity to horizontal polarization leads to uniqueapplications of antennas in seismology that are not available in theprior art.

Velocity information is also obtained from the detected electric field.The arrival of the seismic wave at each electrode is detected when thegenerated EM wave front crosses the first and second electrodes,respectively, and thus the travel time of the seismic wave between theelectrodes can be determined. The detected antenna voltage is alsorelatively insensitive to mechanical coupling of the individualelectrodes to the ground. This provides an advantage over geophones.

The antenna system is highly sensitive to fluids moving at the watertable. If the water table is comparable in depth to the low-velocity orweathered layer (LVL), then the antenna will detect the EM fieldproduced by the seismic wave before the reflected seismic wave traversesthe LVL. Since EM waves are attenuated much less in the LVL than areseismic waves, the EM waves generated below the LVL retain thehigh-frequency information normally lost in the LVL. Also, since the EMsignal is not delayed by the LVL, static velocity shifts are reduced.

An alternative embodiment of the invention includes multicomponentdetection where all three components of the seismic pressure gradientare detected. Another embodiment includes geophones and antennas usedtogether which provides a way to separate source- and receiver-generatedstatic corrections for more effective stacking of seismic data. Inaddition, the difference in arrival times between the electromagneticand seismic signals is used as a measure of the velocity of seismicwaves through the LVL, and attenuation of the seismic signal isdetermined from the frequency content of the two signals. Finally, inyet another embodiment, the electrodes of the antenna are verticallydeployed to detect the vertically polarized component of the seismicenergy in either cross-hole or logging work.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredin conjunction with the following drawings, in which:

FIG. 1 illustrates a basic implementation of electromagnetic groundedantennas as directional geophones;

FIG. 2 illustrates combined use of electromagnetic antennas andgeophones used to measure low velocity layer (LVL) velocity andattenuation of seismic signals;

FIG. 3 illustrates application of electromagnetic antennas to detectmultiple components of signals;

FIG. 4 illustrates an electromagnetic antenna used to detect shear wavesdownhole; and

FIG. 5 illustrates a computer which is used to analyze received dataaccording to the present invention.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Referring now to FIG. 1, a method of operating an electromagneticgrounded antenna as a directional geophone is shown. A source 10 is usedto generate acoustical or seismic waves into the earth, as shown. Thesource 10 may be placed either at or below the surface, as desired. Whenthe seismic wave impacts a formation, P-wave acoustic energy(compressional waves) and S-wave elastic energy (shear waves) areradiated back to the surface as seismic waves.

An electromagnetic grounded antenna 20 according to the presentinvention is used to receive and detect these reflected seismic waves.According to this embodiment, the antenna 20 comprises two electrodes 22and 24 that are placed in the ground and across which a voltage 26 ismeasured. The electrodes 22 and 24 are placed horizontally relative tothe surface, as shown. It is noted that the electrodes 22 and 24comprising the antenna may be placed either on the ground or below theearth's surface, as desired. In a preferred embodiment, the electrodes22 and 24 are placed within a few feet of the surface. This placement ismore convenient for field operation and is thought to provide similardata to buried electrodes. The embodiment shown in FIG. 1 utilizes anelectromagnetic grounded antenna as a directional geophone to measurethe voltage across the two electrodes that is produced by the reflectedseismic waves.

When the seismic waves impact a formation, referred to as a reflector30, both seismic waves and EM waves (not shown) are reflected back tothe surface. EM waves are reflected back to the surface due to the"streaming potential" theory discussed in U.S. Pat. No. 4,904,942, whichis hereby incorporated by reference. According to the "streamingpotential" theory, when the seismic waves impact the reflector 30, apressure gradient is established at the respective depth that pushesdownward on the fluid in the formation in a substantially verticaldirection, thereby breaking molecular bonds between the fluid and theporous surface of the solid formation surrounding the fluid. Thiseffectively establishes a substantially vertical dipole throughout thedepth of the formation and produces a vertical electric field in anupward vertical direction at the point of impact of the seismic waveagainst the reflector. The electric field produces a correspondingelectromagnetic wave that emanates away from the impact region. EM wavesgenerated from the reflector 30 are not shown in FIG. 1.

The seismic wave that is reflected back from the reflector 30 is alsogenerally converted into electromagnetic energy in the neighborhood ofthe antenna 20 near the surface of the earth. This conversion is alsocaused by the "streaming potential" effect. The reflected seismic wavesimpact fluid in the water table 32, causing generation of EM waves, asshown. These generated EM waves are detected by the antenna 20 as aninduced voltage 26.

An electromagnetic wave, unlike a reflected acoustic wave, travels atthe speed of light. Therefore, when the seismic wave impacts thereflector 30 and is converted into an EM wave (not shown), the EM wavereaches the antenna virtually instantaneously. In contrast, thereflected seismic wave requires additional travel time to reach thenear-surface where the water table 32 is located. Therefore, the methoddiscriminates between reflected EM waves from the reflector 30 and EMwaves generated in the vicinity of the antenna, i.e., the water table32, using the time of arrival. EM waves reflected from the reflector 30will reach the antenna 20 in approximately the time required for theseismic wave to travel from the source 10 to the reflector 30. The EMwaves generated in the vicinity of the antenna 20 will reach the antennain approximately the time required for the seismic waves to make a roundtrip from the surface to the reflector 30 and back to the near-surfacewhere seismoelectric conversion again occurs.

Therefore, the operation of an electromagnetic geophone orelectromagnetic antenna to detect the shear component of seismic waves,which is the subject of the present invention, is distinguishable fromconventional electroseismic prospecting, which is concerned with EMwaves generated at a reflector of interest deep below the surface. Whenan EM antenna is being used to detect seismic waves, the antenna 20senses the presence of a reflected seismic or acoustical wave near theearth's surface due to the conversion of the seismic wave into an EMwave within the vicinity of the antenna 20. The vicinity of the antenna20 is defined here as including the LVL, the water table 32, and alsothe shallowest point where seismoelectric conversion can occur.

Referring again to FIG. 1, the antenna 20 is preferably placed a fewfeet (usually 2 to 10 feet) below the earth's surface or on thenear-surface of the earth. The induced voltage 26 measured by theantenna 20 is from the component of the electric field in the directionfrom one electrode to the other. The electric field is generated by theseismic pressure gradient such that ΔP/ΔXαE where ΔP/ΔX is thederivative of the acoustic pressure with respect to distance along theantenna (the difference in pressure at the ends of the antenna dividedby the length of the antenna) and E is the magnitude of the electricfield.

Applicants have discovered that the antenna 20 is sensitive to thecomponent of the seismic pressure gradient in the direction along theantennas' axis. The antenna 20 detects the horizontal component of theseismic pressure gradient and integrates this pressure gradient over thelength of the antenna. Thus the antenna 20 acts as a dispersed detector,which provides increased detection of shear waves over conventionalgeophones, which can only measure received seismic data at one location.This selectivity to horizontal polarization leads to unique applicationsof antennas in seismology, as discussed further below.

To summarize, the antenna 20 detects the horizontally polarizedcomponent of electromagnetic signals resulting from the interaction ofthe reflected seismic (elastic) waves with partially fluid-filledporous, permeable soils and rocks in the neighborhood of the antenna 20.From the voltage measurement 26, the horizontal (shear) component of theseismic pressure gradient can be determined.

Referring now to FIG. 5, a computer comprising system unit 60, monitor62 and keyboard 64 is used to analyze and extract these shear wave datafrom the measured voltage 26 using techniques well known in the art.This includes separating out the compressional wave components and shearwave components using techniques known in the art. Also, the shear waveaspects produced by the seismoelectric conversion at the near surfacecan be separated from the seismoelectric conversion that occurs at thereflector 30 using the time of arrival. The computer is preferablyeither a workstation or personal computer (PC). Alternatively, amainframe computer may be used, as necessary.

The prior art recognized that antennas may provide information similarto that obtained by geophones, although antennas have not been known tohave been used in practical seismic applications. However, theapplication of antennas to detect the horizontal component of theseismic pressure gradient and the fact that antennas integrate thispressure gradient over the length of the antenna is a new discovery.Thus the extraction of shear wave data from voltages measured by anantenna is new in the art.

Electromagnetic grounded antennas provide a higher resolution of sheardata than that previously obtainable. The attenuation of seismic shearwaves by the LVL is usually much more deleterious to signal band widththan for compressional waves. This reduces the utility of seismic shearwave data and thus heretofore shear wave data have not been usedextensively. However, the use of an antenna 20 according to the presentinvention avoids this problem by detecting electromagnetic conversionsfrom up-coming shear waves arriving at the water table, instead ofmerely detecting seismic or acoustic waves at the surface. Since EMwaves are attenuated much less than seismic waves in the LVL,attenuation problems are reduced, as discussed below.

In traditional seismic prospecting, the high frequency components of theseismic wave are attenuated as the seismic wave passes through the LVL.If the water table 32 is comparable in depth to the LVL, then theantenna 20 of the present embodiment will detect the portion of seismicwave energy that converts to EM energy before the seismic wave hastraversed the LVL. If this seismoelectric conversion occurs prior to thereflected seismic wave passing through the LVL, as shown in FIG. 1, theEM wave thus produced will retain the high-frequency information that isnormally lost when the seismic wave traverses through the LVL intraditional seismic prospecting because, in contrast to seismic waves,EM waves are not significantly attenuated in the LVL. Transmission ofelectromagnetic signals depends on the electrical properties of theformation, whereas acoustic signals are transmitted from one rock toanother, and thus acoustic transmission is affected by the contact ofthe rocks. Loosely contacted formations, such as the LVL, result in highseismic signal attenuation and therefore poor signal transmission.Electromagnetic transmission of signals is not affected by this type ofattenuation, and thus EM waves are much less attenuated in the LVL. Inaddition, since EM wavelengths are much greater than the depth of thewater table in soils, attenuation is further reduced.

Velocity information in the detected electric field is also obtainedusing the antenna 20. The first arrival of the seismic wave is detectedwhen the EM wave front crosses the first electrode 22 in the antenna 20while the pulse is broadened by the travel time of the seismic wavebetween electrodes 22 and 24. The travel time between the two electrodes22 and 24, in addition to the depth of the LVL, is thus used todetermine the velocity of the seismic waves.

The detected antenna voltage 26 is only dependent on the motion of porefluids in the region between or in the neighborhood of the electrodes 22and 24 and is relatively insensitive to the mechanical coupling of theindividual electrodes to the ground. This phenomenon provides anadvantage over horizontal geophones where coupling can lead to poorresults. Geophones tend to have very poor acoustic coupling to thesurface since the soil at the surface is generally poorly consolidated.Poor coupling causes high frequency components to be damped out.However, antennas do not have this problem since they detect EM waves,not acoustic waves. In addition, wind noise does not affect operation ofthe antenna 20, whereas wind noise does present problems to horizontalgeophones.

The signal received by the antenna 20 is also averaged over a volume ofthe earth that is comparable to the volume sensed by the electrodes,which might be on the order of 100 m or larger. In contrast, a geophoneis only sensitive to the immediate neighborhood of the ground in whichit is imbedded. Thus, antennas are capable of producing better resultsthan geophones.

The design of the antenna 20, or of the individual antennas used in anantenna array, includes several considerations. First, selection of theappropriate length of an antenna on the surface or along a hole drilledinto the earth is based on conventional seismic-style considerationssuch as: 1) the antenna must be long enough compared to the expectedwavelength of the field that it is detecting, 2) there must be adetectable signal (potential difference) between endpoint electrodes,i.e., the electrodes have to be far enough apart so that there is asignificant difference in the voltage at the two ends of the antenna;otherwise, the electric field is too small to be detected; and 3) theantenna must not be too long that it aliases wavelengths that arerelevant to the imaging spectrum for the data. Another importantconsideration is the selection of the antenna and electrode material.These materials can be based on conventional considerations developed inelectromagnetic and magnetotelluric geophysics.

When using arrays of antennas, the arrays are preferably designedaccording to standard electromagnetic geophysics and/or radar methodsfor maximizing signal to noise ratios through techniques such asstacking of signals and noise-cancelling array widths, as is well knownin the art. Another consideration is the selection of signalamplification and recording systems. This selection is based onappropriate combination of existing high fidelity amplifiers and seismicrecording technology. Care should be taken to reduce transientelectromagnetic noise in the system using devices such as a DC batterypower supply rather than an AC generator as the source of power for therecording system and taking care to ground the recording system at apoint several times more distant from the antennas than the antennalength.

Referring now to FIG. 2, an electromagnetic grounded antenna 20 inconjunction with geophones 40 may also be used in performing staticcorrections. The primary conversions of seismic signals intoelectromagnetic signals that have been observed on antennas in fieldexperiments arise from the shallowest water table layer. Accordingly,recorded signals have receiver statics (shifts in event arrival timesdue to near-surface effects) for antenna geophones that are datumed tothe water table (which is also usually the shallowest seismic refractinglayer and is used for refraction statics corrections in seismicprocessing), whereas conventional mechanical geophones are datumed tothe surface of the earth. The topographic relief of the surface isgenerally greater and the seismic velocity variability of near-surfacematerials between receiver locations is also greater than for the watertable. Consequently, antennas need much smaller receiver staticadjustments than conventional geophones. Due to this difference instatics, combined use of geophones and antennas allows separation ofsource- and receiver-generated static corrections for more efficientstacking of seismic data.

An electromagnetic grounded antenna 20 and several geophones 40 are usedin the embodiment of FIG. 2 to measure the velocity of signals travelingthrough the near surface layer (low velocity layer--LVL) and also tomeasure the attenuation of signals in the LVL. This method comprisessimultaneous recording of signals from upcoming seismic waves directlyon the surface geophones 40 and their electromagnetic conversion at thewater table on co-located or negligibly-offset antennas 20. The depthfrom the surveyed surface geophone locations to the water table ispreferably determined beforehand by seismic refraction statics or duringfield work by shot-hole drilling observations or by other standardmeans.

As previously noted, electromagnetic waves travel at the speed of light,while seismic waves travel at the speed of sound, and thus EM wavestravel at a much faster rate than seismic waves. Because of the vastdifference in the speeds of the two waves, generated EM waves reach theantenna in approximately the travel time of its derivative seismic wave.The arrival time difference of the seismic and electromagnetic signalsindicates the seismic wave travel time through the LVL, since the speedof electromagnetic propagation through the LVL is virtuallyinstantaneous compared to the speed of elastic wave propagation. The LVLdepth divided by the travel time provides the seismic velocity. Inaddition, comparison of the frequency spectra of the signals detected onthe antenna versus the geophone indicates relative signal attenuationdue to the LVL.

Alternate embodiments of this invention can also be implemented inspecific applications. Referring now to FIG. 3, one embodiment uses anumber of antennas in multicomponent detection. In this embodiment,three non-parallel antennas are used at each multicomponent signaldetection station to detect the X, Y, and Z components of the electricfield, and thus all three components of the electric field are detected.For example, at each station, a vertical antenna 20Z approximately threefeet long and two orthogonal horizontal antennas 20Z and 20Y of lengthequal to station spacings set by anti-aliasing considerations(approximately 100 feet long and possibly of unequal lengths) provide asensor system to detect the three orthogonal components of mechanicalmotion affecting relative fluid/rock motions. These signals arepreferably each recorded as seismic-like amplitude versus time tracesand are processed for imaging in the same way as would be done forhorizontal geophones because polarity flips in amplitudes for in-line(SV-wave) horizontal components, as one passes from one side of theseismic source to another, is the same for in-line horizontal antennasas for in-line horizontal geophones. In addition, as previouslymentioned, attenuation of seismic waves by the near-surface is avoidedbecause the antennas detect seismic waves converted to electromagneticsignals at the water table.

FIG. 4 illustrates vertical seismic profiling (VSP) using two sources 10and 12 and one antenna 20 according to the present invention in whichdetection of shear is desired. This embodiment utilizes verticallydeployed electrodes to detect the vertically polarized component ofshear wave energy converted to electromagnetic signals either at theborehole wall or conducting fluid insulating or fluid-gas interfaces innearby rock formations. It is also noted that electrodes deployed inhorizontal arrays, either suspended in the borehole fluid or in contactwith the borehole wall, can detect the horizontally polarizedcomponents. It is contemplated that virtually any type of arrangement ofsources and detectors can be used, including cross-borehole.

In addition, arrays of antennas may be used like geophones to selectparticular components of the pressure field, to reject an unwantedsignal, to stack a signal for improved signal-to-noise ratio, and todirectly measure velocity and attenuation. With the use of sophisticateddetection techniques, such as using arrays of antennas analogous togeophone arrays, antennas can achieve sensitivity comparable togeophones.

Therefore, the use of antennas according to the present invention offerscertain advantages over geophones. First, because antennas arepreferentially selective in the direction in which they are aligned,they detect the shear component of seismic waves. The detected EM signalis distributed over the length of the antenna, and thus improved shearwave data are obtained. Also, antennas do not have horizontal couplingproblems and, therefore, supply enhanced directional and temporalderivative information. Antennas are also particularly useful inreducing static and noise effects in shear-wave-prospecting and indirect measurements of the low-velocity-layer (LVL) velocity andattenuation. These features enable the collection of more reliablehigh-resolution shear-wave land data.

Having described the invention above, various modifications of thetechniques, procedures, material and equipment will be apparent to thosein the art. It is intended that all such variations within the scope andspirit of the appended claims be embraced thereby.

What is claimed is:
 1. A method of seismic prospecting usingelectromagnetic grounded antennas, comprising the steps of:placing anantenna comprising at least two electrodes at or below the earth'ssurface; initiating a seismic wave into the earth from a source locationsuch that the downgoing seismic wavefront encounters a reflector and isreflected back to the surface as reflected seismic waves; detectingelectromagnetic waves generated by the reflected seismic waves in thevicinity of the antenna; measuring the induced voltage across theantenna; and analyzing the induced voltage to determine the horizontalcomponent of the reflected seismic wavefront.
 2. The method of claim 1,wherein said step of placing includes placing said two electrodeshorizontally relative to the surface.
 3. The method of claim 1, whereinsaid step of placing includes placing an array of antennas at or belowthe earth's surface.
 4. The method of claim 1, wherein said antenna isplaced so as to be spaced apart from said seismic source.
 5. The methodof claim 1, wherein said step of placing includes placing said antennabelow the earth's surface.
 6. The method of claim 1, wherein said sourcelocation is located below the earth's surface.
 7. The method of claim 1,wherein said step of placing includes placing said antenna below theearth's surface; andwherein said source location is located below theearth's surface.
 8. The method of claim 1, wherein said step of placingincludes placing said antenna on the earth's surface.
 9. The method ofclaim 1, wherein said source location is located on the earth's surface.10. The method of claim 1, wherein said step of placing includes placingsaid antenna on the earth's surface; andwherein said source location islocated on the earth's surface.
 11. The method of claim 1, wherein saidstep of analyzing the induced voltage includes separating the voltageresulting from compressional and shear wave components of the reflectedseismic wavefront by using the time of arrival of the electromagneticwaves.
 12. The method of claim 1, wherein said step of analyzing theinduced voltage includes separating the voltage resulting fromelectromagnetic waves generated by the reflected seismic wavefront inthe vicinity of the antenna from the voltage resulting fromelectromagnetic waves generated by the reflector by using the time ofarrival of the electromagnetic waves.
 13. A method of determiningseismic wave velocity, comprising the steps of:placing an antennacomprising first and second electrodes at or below the earth's surface,wherein said electrodes are positioned horizontally and said secondelectrode is placed at a selected distance from said first electrode;initiating a seismic wave into the earth from a source location suchthat the downgoing seismic wavefront encounters a reflector and isreflected back to the surface as reflected seismic waves, wherein saidreflected seismic waves generate electromagnetic waves in the vicinityof the antenna; determining the time of arrival of said electromagneticwaves at said second electrode; and determining the velocity of saidreflected seismic waves by calculating the ratio of said distancebetween said first and second electrodes to the difference in times ofarrival of said electromagnetic waves at said electrodes.
 14. The methodof claim 13, wherein said step of placing includes placing said antennabelow the earth's surface.
 15. The method of claim 13, wherein saidsource location is located below the earth's surface.
 16. A method ofseismic prospecting using electromagnetic grounded antennas, comprisingthe steps of:placing three antennas at or below the earth's surface,wherein said antennas are placed orthogonally relative to each other;initiating a seismic wave into the earth from a source location suchthat the downgoing seismic wavefront encounters a reflector and isreflected back to the surface as reflected seismic waves; said threeantennas detecting electromagnetic waves generated by the reflectedseismic waves in the vicinity of said antennas; measuring the inducedvoltage across each of said antennas; and analyzing the induced voltageto determine the X,Y, and Z components of the reflected seismic waves.17. A method of seismic prospecting using electromagnetic groundedantennas, comprising the steps of:placing an antenna comprising firstand second electrodes in the borehole of a well, the electrodes beingspaced apart; initiating a seismic wave into the earth from a sourcelocation; detecting electromagnetic waves generated by the seismic waveby measuring an induced voltage across the antenna; and analyzing theinduced voltage to determine the component of the seismic wave which isparallel to the antenna.
 18. The method of claim 17 wherein the firstand second electrodes are vertically deployed in the well.
 19. Themethod of claim 17 wherein the first and second electrodes are deployedin a horizontal array in the well.
 20. The method of claim 17 whereinthe first electrode is deployed in a first borehole and the secondelectrode is deployed in a second borehole.